InDesign not pulling Heading1 style for table of contents for threaded text frames

Hello,

Maybe someone can help me -

I have laid out a book (not with stories, just 156 pages).

I have Heading1 style applied to the chapter headings throughout the book.

The text was imported from a Word doc, so the majority of the book is threaded text boxes.

After laying out the whole book, I go to generate the TOC.

For some reason, the only heading getting pulled is the Endnotes, which automatically generated a new text box. It has the exact same paragraph style applied. I tried a few other things other peope tried to fix their TOC, including saving the file as an IDML, and deleting the extra text boxes I had to separate out the Chapter Headings, but still no joy. If I make a new (unthreaded) text box and apply Heading1 style to the text, THOSE show up in the TOC. I really don't want to have to make a TOC manually, and add jump links manually, because chances are my client will have edits and I'll have to redo it again. Any leads would be most appreciated!